I've seen a number of posts inquiring about removing all data from a feed but keeping the feed, but I've never seen it answered. Is there a way to do it via GUI or from a command line (via curl, for example)?

It comes up frequently for me - like I make a change to the program and then I have all this stale data and the plot scale is goofy or I move the sensor from testing into 'production' and the same thing happens. Removing the entire feed and recreating it isn't convenient.

There's no way to remove all data from a feed at once. Feed deletion is an API action (https://io.adafruit.com/api/docs/#delete-data-point) and is therefore limited to your account's data rate (30 or 60 data points per minute for IO Free and IO Plus, respectively).